Transcript What is DNA, and How is it Used in Today’s Society?

What is DNA, and How is it Used in
Today’s Society?
• Deoxyribonucleic Acid (DNA)
– DNA is found in all living things (all life related?)
– The hereditary material; found in the nucleus of
eukaryotes (copied before each cell division; passes
codes for physical traits to offspring)
– Today, segments of DNA (genes) can be manipulated,
and can be removed from/inserted into organisms
(biotechnology, transgenic organisms)
– Your DNA code is unique (excl. identical twins) 
criminal and paternity applications
– Genetic diseases linked to various genes  genetic
screenings and counseling
What is the Structure of DNA, and How
is it Copied Before Cell Division?
• Structure of DNA
– A polymer, composed of nucleotides (which consist of
a sugar, a phosphate group, and a nitrogenous base)
• Sugar is deoxyribose
• Nitrogenous bases: guanine, cytosine, adenine, and thymine
– Double-stranded molecule, wound in helix (Watson,
Crick, and Wilkins  Nobel Prize)
• Two strands joined by hydrogen bonds (two bonds between
T/A; three bonds between C/G); unzip at high
temperature or via enzyme action
• DNA Replication (occurs during S-phase)
– Code of new strand based on original template
– Enzymes involved: DNA Polymerase, Helicase
Figures 5.13 and 5.16
Figure 5.17
What is the Role of RNA in Gene
Expression?
• Gene Expression:
Gene (DNA)  message (m-RNA)  polypeptide (protein)
TRANSCRIPTION
TRANSLATION
• Transcription (gene  m-RNA)
– Occurs in nucleus
– Nucleotide sequence of m-RNA based on code of DNA
(gene)
• RNA polymerase enzyme involved in process
– In eukaryotes, m-RNA often edited into exons and
introns; exons processed into mature m-RNA that
enters cytoplasm and is used for protein synthesis
Fig. 5.18
How are Proteins Synthesized
Based on Genetic Instructions?
• Translation (Protein synthesis: m-RNA 
polypeptides)
– Occurs at ribosomes (in rough ER or cytoplasm)
– t-RNA, bound to amino acids, associates with ribosome
– Order of amino acids determined by GENETIC CODE:
m-RNA codons (base triplets) bind to anticodons of
t-RNAs; amino acids join (peptide bonds) to form
polypeptides
– Polyribosomes found in cells that exhibit high levels of
protein synthesis (when many copies of same polypeptide are routinely synthesized)
Figures 5.21
and 5.22
Table 5.3
What are Chromosomes, Genes,
Genomes, and Karyotypes?
• Chromosomes and Genes
– Chromosome: genetic material (DNA) packaged with
histone proteins in discrete units (nucleosomes)
• Centromere joins two chromatids; can be central or off-center
• Categorized by size, centromere location, and band patterns
– Gene: nucleotide sequence that codes for a functional
polypeptide (affect traits), as opposed to “junk DNA”
• Genome
– The entire genetic sequence of an organism (all chromosomes, all nucleotides); chromosome numbers vary by
species
• Karyotypes
– Photograph of entire set of an organism’s chromosomes
– Used for diagnosis of chromosomal abnormalities
What are the Differences Between Sexual
and Asexual Reproduction?
• Asexual Reproduction: offspring are clones of a
single parent; variation among individuals
limited to mutations
– Bacteria divide by binary fission
– Spores in many eukaryotes (environmentally resistant
reproductive cells that can develop alone)
• Sexual Reproduction: two parents donate genes to
offspring via gametes (sex cells)
– Gametes are haploid, must fuse  diploid zygote
– Several sources of variation in addition to mutation 
great physical diversity among individuals’ traits
• Many organisms with both asexual and sexual
cycles (alternate, depending on various factors)
– Example: parthenogenesis in some fishes and reptiles
RED CLONE
YELLOW CLONE
ORANGE CLONE
How are Gametes Produced? What are Some
Sources of Variation Among Individuals?
•
Gametogenesis: formation of gametes from
somatic cells (via MEIOSIS, cell differentiation)
–
Spermatogenesis: formation of sperm cells; occurs in testes
•
–
Oogenesis: formation of egg cells (oocytes); occurs in ovaries
•
•
Haploid spermatids differentiate into sperm cells (with cap and
flagellum)
One of four grand-daughter cells absorbs cytoplasms of others (egg
cells are very large cells, with RNA-rich and protein-rich
cytoplasms); polar bodies remain after formation of oocyte
Important sources of variation for sexually
reproducing organisms (other than mutation):
1.
2.
3.
4.
5.
Independent assortment of chromosomes in meiosis
Crossing over (genetic recombination) among homologous
chromosomes; more likely to occur away from centromere
Gene duplications, inversions, translocations, and deletions
Non-disjunction of chromosomes / duplication of chromosomes
Whole-genome duplications
Figure 5.2
Fig. 5.10
What are the Two Laws of Mendelian
(Classical) Genetics? What are Alleles?
• Developed by Gregor Mendel (1822-1884): studied
heredity in pea plants (mainly texture and color of
seeds); based solely on observations (no knowledge of
DNA or meiosis) – see cartoon
– Law of Segregation: there are two sets of genes for a particular trait
(one from each parent), but only one gets into gamete during
gametogenesis
– Law of Independent Assortment: during gametogenesis, a gene
that enters a gamete does so independently of those for other
traits (ex. if red hair expressed, blue eyes not necessarily
expressed)
• Alleles: different forms of same gene (found at same locus)
– Dominant allele: the form expressed in offspring (if present)
– Recessive allele: masked by dominant allele (not expressed if
dominant allele present), but can still be passed on to next
generation (by a carrier)
What are Genotypes and Phenotypes?
How do we Solve Genetics Problems?
• Phenotype: description of form of physical trait an individual
exhibits (ex. trait of hair color, “red hair” is a phenotype)
• Genotype: description of individual’s condition at the genetic
level; three possible genotypes:
– Homozygous dominant (AA): both genes for trait instruct to produce
dominant phenotype
– Homozygous recessive (aa): both genes for trait instruct to produce
recessive phenotype
– Heterozygous (Aa): genetic instructions conflict; for Mendelian traits,
dominant phenotype results (recessive masked)
• Solving Genetics Problems: Mendel described traits in
P (parental) generation and F1, F2 (filial) generations
– Monohybrid Cross: single trait; parents’ genotypes crossed using
Punnett Square
– Dihybrid Cross: two traits; find results for each single trait with
Punnett Square, then multiply probabilities (ex. ¼ X ¼ = 1/16)
Figure 5.6
What are Some Modern Additions to
Mendelian Genetics?
• Polygenic traits: traits caused by multiple genes
– Variation in population often follows bell curve when frequency is
plotted against measurement of phenotype (ex. height)
• Multiple alleles: ex. blood types
– Only two alleles in any cell, but more than two in population
• Linked genes: loci typically in close proximity
• Incomplete Dominance (and Co-dominance)
– Phenotype for heterozygous genotype is a mixture (blend) of
those caused by homozygous genotypes
– Problem-solving is the same as Mendelian traits; need to
evaluate genotypes differently; any mention of three phenotypes in problem?
• Examples: color of petals in roses; Sickle-cell anemia
Figure 5.5
How is Sex Determined? What are
Some Examples of Sex-linked Traits?
• Sex Determination: 23rd pair of human chromosomes are the sex chromosomes, others are
autosomes - see cartoon
– Females: XX; Males: XY
• Sex-linked traits: X-sex chromosome has many
genes other than those for sex determination,
but Y-sex chromosome does not  no male
carriers for sex-linked traits
– Examples: color blindness, hemophilia
– Same methods to solve problems, but must account for
sex of parents and offspring [ex. XHXh x XHY  XHXH,
XHY, XhXH (= XHXh), XhY]
Figures 5.3 and 5.4